Nickel aluminides can be formed by thermal annealing of alternate layers of Ni and Al that react exothermically. These metals have medium/high energy of mixing and when in adiabatic conditions the reaction could become self-propagating. The first phase to form in Ni/Al reactions has been widely investigated, but there is no consensus regarding this subject. Depending on the overall chemical composition and modulation period of the multilayer, and on the processing history, the first phase to form could be NiAl₃, NiAl and Ni₂Al₉.